1. Field of the Invention
This invention relates to a fluoride glass, which is suitably used as low loss optical fiber, infrared optical fiber for thermometer, infrared laser window, etc.
2. Description of the Prior Art
Infrared transmitting glass has a wide scope of use such as infrared optical fiber for thermometer, infrared laser window, low loss optical fiber, etc., so that efforts are being made for further development of such glass. As the infrared transmitting glass used for such purposes, there have been known ZrF.sub.4 /BaF.sub.2 -based fluoride glass and AlF.sub.3 -based fluoride glass as well as fluorophosphate glass.
Among said known types of infrared transmitting glasses, the ZrF.sub.4 /BaF.sub.2 -based fluoride glass has the disadvantage that it is low in hardness and liable to damage or scratch, so that when it is used for instance as low loss optical fiber, the fiber tends to be damaged and is subject to the risk of being broken at the damaged section. Further, this fluoride glass is poor in chemical durability, and it absorbs moisture from the atmosphere at normal temperature to produce OH bond which is a causative factor of infrared absorption, thus presenting the problem in reliability.
The AlF.sub.3 -based fluoride glass has the drawback that the composition has a high tendency for crystallization, so that the molten glass composition must be cooled rapidly to form the glass. It is also impossible to obtain large-sized glass, and thus the scope of use of this fluoride glass is limited.
The fluorophosphate glass has the problem that its light transmittability is limited to near infrared region (wavenumber to around 2400 cm.sup.-1), and further it has infrared absorption at the wavenumber of around 3200 cm.sup.-1 due to OH group that remains in the glass.
The primary object of this invention, therefore, is to provide a fluoride glass which has little likelihood of crystallization (or devitrification . . . hereinafter referred to as simply "crystallization") in the production thereof, and which also has excellent chemical durability and high surface hardness.
Another object of this invention is to provide a fluoride glass minimized in infrared absorption due to OH group.
Other objects of this invention will become apparent from the following description and the drawing.